U0101
Lost Communication with TCM
The ECU has stopped receiving messages from the transmission control module. The transmission falls into limp-home mode (typically locked in 3rd or 4th gear), the speedometer may stop working, and shift adaptation freezes.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: hard
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
Ordered by how often each turns out to be the cause.
- TCM unpowered or grounded poorlyCommon
- CAN wiring to TCM brokenPossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- TCM
- CAN Bus Wiring
U0101 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.